Application of silver nanofluid containing oleic acid surfactant in a thermosyphon economizer
 Thanya Parametthanuwat^{1},
 Sampan Rittidech^{1}Email author,
 Adisak Pattiya^{2},
 Yulong Ding^{3} and
 Sanjeeva Witharana^{3}
DOI: 10.1186/1556276X6315
© Parametthanuwat et al; licensee Springer. 2011
Received: 13 October 2010
Accepted: 7 April 2011
Published: 7 April 2011
Abstract
This article reports a recent study on the application of a twophase closed thermosyphon (TPCT) in a thermosyphon for economizer (TPEC). The TPEC had three sections of equal size; the evaporator, the adiabatic section, and the condenser, of 250 mm × 250 mm × 250 mm (W × L × H). The TPCT was a steel tube of 12.7mm ID. The filling ratios chosen to study were 30, 50, and 80% with respect to the evaporator length. The volumetric flow rates for the coolant (in the condenser) were 1, 2.5, and 5 l/min. Five working fluids investigated were: water, waterbased silver nanofluid with silver concentration 0.5 w/v%, and the nanofluid (NF) mixed with 0.5, 1, and 1.5 w/v% of oleic acid (OA). The operating temperatures were 60, 70, and 80°C. Experimental data showed that the TPEC gave the highest heat flux of about 25 kW/m^{2} and the highest effectiveness of about 0.3 at a filling ratio of 50%, with the nanofluid containing 1 w/v% of OA. It was further found that the effectiveness of nanofluid and the OA containing nanofluids were superior in effectiveness over water in all experimental conditions came under this study. Moreover, the presence of OA had clearly contributed to raise the effectiveness of the nanofluid.
Introduction
An economizer is a type of heat exchanger that can be classified into four types: tubular heat exchanger type (double pipe, shell and tube, and coil tube), plate heat exchanger type (gasketed, spiral, plate coil, and lamella), extended surface heat exchanger type (tubefin and platefin), and regenerator type (fixed matrix and rotary) [7–9]. Nada et al. [10] used a TPCT in a solar collector with a shell and tube heat exchanger and observed a uniform temperature distribution [10]. The performance of a TPCT depends upon the aspect ratio (length to diameter) and the filling ratio (volume of fluid to volume of evaporator). Another application of the TPCT is in the energy recovery systems in air conditioning plants in tropical countries. There, the inlet air is precooled by the cold exhaust stream before it enters the refrigeration equipment [11–13]. Lukitobudi et al. [14] studied the heat exchange from hot water to air using a TPCT, and Atipong et al. [15] studied oscillating heat pipe in a wireontube heat exchanger. The results obtained by both groups showed that after the heat recovery, the effectiveness and heat transfer of the evaporator and condenser increased by about 48%. Mostafa et al. [16] reported that the economizer in the TPCT imposed limitation to the heat transfer due to the lower quality of the working fluid accumulated inside. When nanofluids were used as working fluids, they increased the thermal and heat transfer capacities. Nanofluids are created by suspending ultrafine metallic or nonmetallic particles typically of several tens of nanometers in size, in base fluids such as water, oil, and ethylene glycol. Nanofluids were known to have enhanced the thermal conductivity and convective heat transfer. However, to obtain a sizable enhancement in thermal conductivity, the particle volume concentration needs to be significantly large, in the order of 0.5 vol% or above [17, 18]. The distinct features of nanofluids are their stronger temperaturedependent thermal conductivity than the base fluid [19, 20]. The thermal conductivity also depends upon the concentration of the added surfactant. In some instances, the nanofluids were unstable and the nanoparticles found to have precipitated. A surfactant improves the stability of a nanofluid by uniform dispersion of particles [21–23]. A surfactant can adsorb gas in a liquidgas interface and decrease the interfacial tension. Some surfactants may flocculate in the bulk solution [24, 25].
The TPEC used in this study was a special type that uses nanofluids in the thermosyphon to transfer heat from evaporator to condenser without external energy requirement. The primary objective of this study is to design and test the TPEC that will increase the heat transfer to water. The heat will be helpful to increase effectiveness of the TPEC. This TPEC was designed using a correlation of Kutateladza number (Ku).
TPEC design, experimental apparatus, and analysis
TPEC design
System design conditions
Section of economizer  Condition design 

Length was 250 mm  
Evaporator section  Hot water flow was 80°C 
Volumetric flow rate was 5 l/min  
Adiabatic section  Length was 250 mm 
Length was 250 mm  
Condenser section  Cool water flow was 25°C 
Volumetric flow rate was 1 l/min 
The calculations showed that the number of tubes for TPEC is 12.
Experimental apparatus
Controlled and variable parameters
The tubes were arranged in a staggered  

Operating temperature of 60,70 and 80°C  
The controlled parameters  Silver nanofluid concentration of 0.5 w/v% 
Volumetric flow rate was 5 l/min in evaporator section  
Cool water flow was 25°C in condenser section  
Working fluid = pure water, silver nanofluid concentration of 0.5 w/v% and silver nanofluid concentration of 0.5 w/v% mixed oleic acid surfactant  
The variable parameters  Concentration of oleic acid surfactant were 0.5, 1, 1.5 w/v% 
Volumetric flow rate were 1, 2, 5 l/min in condenser section  
Filling ratio = 30, 50, and 80% (by total length of evaporator) 
The nanofluid was produced by suspending metal or metal oxide nanoparticles in a base fluid such as water. The preparation involved several steps such as changing the pH value of the suspension, using surfactant activators, and using ultrasonic vibration. For this study, the nanofluid was sonicated for 5 h in ultrasonic bath. Silver nanopowder (<100 nm particle size, 99.9% metals basis) and oleic acid were obtained from SigmaAldrich Inc, Milwaukee, Wisconsin: USA. The silver nanoparticles were suspended in DI water with concentrations of 0.5 w/v% [16]. After that, the silver nanoparticles were suspended into deionized water with concentrations of 0.5 w/v% mixed with oleic acid surfactant concentration of 0.5, 1, and 1.5 w/v%, respectively. The nanofluids were stable for a long time.
The effectiveness analysis
To analyze the performance of the TPEC, the effectiveness (ε) was calculated by the Number of Transfer Unit Method (ε  NTU). The NTU is based on the heat exchanger effectiveness defined as the ratio of actual heat transfer in a heat exchanger to the maximum possible amount of heat that could be transferred with an infinite area [26].
The experimental conditions are given in Table 2.
Result and discussion
Effect of operating temperature on heat flux
Effect of filling ratios on heat flux
Effect of volumetric flow rate on heat flux
Effect of concentration on effectiveness
Effect of operating temperature on effectiveness
Effect of filling ratios on effectiveness
Effect of volumetric flow rate on effectiveness
Conclusions
A TPEC was designed using a correlation of Kutateladza number (Ku) for the prediction of heat transfer of the TPCT. Experiments were conducted on the TPEC using various working fluids to study the effects of various parameters on the heat flux and the effectiveness. It was found that pure water gave the lowest values for heat flux, whereas the silver nanofluid and the silver nanofluid containing oleic acid gave the higher heat fluxes. In particular, the silver nanofluid containing 1 w/v% oleic acid exhibited the best performance in all experiments. Moreover 80°C operating temperature, 50% filling ratio, 5 l/min volumetric flow rate were proved to be the optimum working conditions that yielded the maximum heat flux from this TPEC. Furthermore, it was found that the highest value for effectiveness was also displayed by the silver nanofluid containing 1 w/v% oleic acid at 80°C operating temperature, 50% filling ratio, and 1 l/min volumetric flow rate.
List of symbols
A Total heat transfer area, surface area of evaporator (m^{2})
C Capacity rate (kJ(s°C)^{1})
C_{ p }Specific heat capacity constant pressure, (J(kg °C)^{1})
D Diameter (m)
h_{ fg }Latent heat of vaporization, (kJ · kg^{1})
k Thermal conductivity (W/mK)
L Length of thermosyphon (mm)
L_{c} Characteristic length (m)
NF Silver nanofluid
NF + OA Silver nanofluid with oleic acid
NF + OA 0.5 w/v% Silver nanofluid with oleic acid concentration 0.5 w/v%
NF + OA 1 w/v% Silver nanofluid with oleic acid concentration 1 w/v%
NF + OA 1.5 w/v% Silver nanofluid with oleic acid concentration 1.5 w/v%
OA Oleic acid
Q Heat transfer rate (W)
q Heat flux (kW/m^{2})
T_{out} Outlet temperature at condenser section (°C)
T_{in} Inlet temperature at condenser section (°C)
T_{ v }Operating temperature (°C)
ΔT Temperature difference (°C)
U Overall heat transfer coefficient (W · m^{2} · K)
V Velocity (m · s^{1})
Greek symbols
ρ Density (kg · m^{3})
μ Viscosity (Pa · s)
σ Surface tension (N · m^{1})
ε Effectiveness of economizer
Subscripts
a Adiabatic
c Condenser, cold fluid
e Evaporator
h Hot fluid
i in
l Liquid
max Maximum
min Minimum
o out
v Vapor
Ar, Archimedes number =
Bo, Bond number =
Co, Condensation number =
Ja, Jacob number =
Ku, Kutateladza number =
Aspect ratio =
Pr, Prandtl number =
Pe, Peclet number =
Cd, Drag number =
Z, Ohensorge number =
Abbreviations
 NF:

nanofluid
 OA:

oleic acid
 TPEC:

thermosyphon for economizer
 TPCT:

twophase closed thermosyphon.
Declarations
Acknowledgements
Financial support from the Thailand Research Fund through the Royal Golden Jubillee Ph.D. Program (Grant No. PHD/0340/2550) to TP and SR is acknowledged. TP, SR were also supported generously by the Faculty of Engineering, Mahasarakham University, Thailand and Institute of Particle Science & Engineering, University of Leeds, United Kingdom.
Authors’ Affiliations
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